The present invention relates generally to an electronic device, and more particularly, to a semiconductor memory device and an operating method thereof.
Semiconductor devices, in particular semiconductor memory devices, are classified into volatile memory devices and nonvolatile memory devices.
The nonvolatile memory devices can maintain data stored therein even when supply of power is interrupted, although read and write speeds are comparatively low. Therefore, the nonvolatile memory device is used when there is the need for storing data which must be maintained regardless of supply of power. Representative examples of the nonvolatile memory device include a read-only memory (ROM), a mask ROM (MROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a flash memory, a phase-change random access memory (PRAM), a magnetic RAM (MRAM), a resistive RAM (RRAM), a ferroelectric RAM (FRAM), etc. The flash memory is classified into a NOR type memory and a NAND type memory.
Flash memory devices may be classified into a two-dimensional semiconductor device, in which strings are horizontally formed on a semiconductor substrate, and a three-dimensional semiconductor device in which strings are vertically formed on a semiconductor substrate. Three-dimensional semiconductor memory devices are memory devices that overcome the limitation in the degree of integration of two-dimensional semiconductor memory devices, and include a plurality of strings that are vertically formed on a semiconductor substrate. Each string includes a drain select transistor, a plurality of memory cells, and a source select transistor that are coupled in series between a bit line and a common source line (SL).